The present invention generally relates to electrochemical cells (batteries) and, more particularly, relates to an alkaline electrochemical cell having a pressure relief vent formed in the cell container for effectively venting gases at excessive pressure.
Alkaline electrochemical cells employ a container typically in the form of a cylindrical steel can having a closed bottom end, an open top end, and a cylindrical side wall extending between the top and bottom ends. Contained within the can is a positive electrode, also referred to as the cathode, which typically comprises manganese dioxide. Also contained within the can is a negative electrode, also referred to as the anode, which typically comprises zinc. In bobbin-type cell constructions, the cathode may be ring molded or impact molded against the interior surface of the steel can, while the anode is generally centrally disposed within the can. A separator is located between the anode and the cathode, and an alkaline electrolyte solution contacts the anode, cathode and separator. A conductive current collector is inserted into the anode active material. A collector and seal assembly, which typically includes an annular polymeric seal, an inner metal cover, the current collector, and an outer cover, provides closure to the open top end of the steel can to seal closed the steel can.
Conventional alkaline electrochemical cells are commonly sealed closed by placing the collector and seal assembly with the annular polymeric (e.g., nylon) seal in the open end of the steel can and crimping the upper end of the can radially inward and over the outer periphery of the seal to compress the seal against the can. The electrochemically active materials, such as zinc, may generate hydrogen gas and other gases. With the can sealed closed, excessive build-up of high pressure gases within the sealed can may lead to damage to the cell and/or the device in which the cell is employed. Thus, it is desirable to provide a controlled vent mechanism that vents highly pressurized gases from within the can to prevent the pressurized gases from reaching excessive levels that may cause the can to uncrimp and release excessive electrolyte solution and particulate matter.
A common approach to venting excessive pressurized gases from within an electrochemical cell includes the use of a vent formed in the annular polymeric seal of the collector and seal assembly, which is intended to rupture upon experiencing excessive pressure within the sealed volume of the cell. One example of a vent formed as a thin portion in an annular polymeric seal is disclosed in U.S. Pat. No. 5,667,912, with the vent intended to shear when the pressure exceeds a predetermined pressure limit. The conventional approach of employing a vent in the seal structure requires an assembly that generally consumes a significant amount of useable volume within the battery can. This results in less space available for the electrochemically active materials, thus limiting battery service life capability.
In order to minimize space occupied by the collector and seal assembly, it has been proposed to form the pressure relief vent mechanism in the closed bottom end wall of the metal can, and to cover the vent with the positive contact terminal. Examples of a vent and contact terminal provided on the closed bottom end wall of the battery can are disclosed in U.S. Pat. No. 6,620,543 and U.S. Patent Application Publication No. 2004/0157115 A1, the entire disclosures of which is hereby incorporated herein by reference. According to these approaches, the pressure relief vent, formed as a reduced thickness groove in the bottom end wall of the metal can, is formed in one or two semicircular C-shapes generally centered about the central location of the bottom closed end of the can. When the internal pressure exceeds a predetermined limit (relative to the outside atmospheric pressure), the vent ruptures to release pressure from within the internal volume of the battery can to the outside atmosphere. The previous proposed C-shaped vents may, in some situations, require a thin coin thickness, such as 2.0 mils, to yield an acceptable vent pressure. Such thin vents may be susceptible to damage such as during cell manufacturing (e.g., impact molding) and therefore may be unacceptable for some cells.
Typically, welded onto the closed bottom end wall of the conventional battery can is the positive contact terminal or cover which includes an outwardly protruding nubbin having an upstanding wall extending from a peripheral flange that is welded to the closed bottom end wall of the can. Conventionally, the peripheral flange is spot welded to the steel can via three symmetric welds, spaced apart from each other at equal distances, i.e., sequentially located at angles of one hundred twenty degrees (120°). In some proposed batteries, the positive contact terminal is supposed to allow gas to escape between the peripheral flange of the contact terminal and the bottom end wall of the can between adjacent welds. However, due to bulging of the can and resultant flexing of the bottom end wall, and further due to improved low profile walls, and the symmetric spacing of the adjacent welds (e.g., 120°), the peripheral flange of the overlying cover may form a seal against the bottom end wall of the can and prevent proper venting of gas to the outside environment. Thus, proper venting of excessive gases may be inhibited which could lead to a possible crimp release.
Accordingly, it is desirable to provide for an electrochemical cell having an effective vent formed in the battery can. It is further desirable to provide for a battery can that vents excessive gases and has a cover that allows the excessive gases to be effectively released to the outside environment.